[2.4] Support multi document properties files

[AlexFalappa]

Spring Boot 2.4 introduces a YAML like syntax to split a configuration properties file into multiple sections treated as separate documents.

See this blog article for a summary.

[AlexFalappa]

See the reference guide here.

This is not an easy task as the Abstract Syntax Tree of configuration properties files is currently single-document and assumes a java.util.Map behaviour, duplicates override their values.